1. Field of the Invention
This invention relates to an improvement for a vibrating cylinder pressure transducer, and, more particularly, to an improvement which compensates for the decline in the stability of such a pressure transducer as a result of increasing temperature.
2. Description of the Prior Art
Vibrating cylinder pressure transducers are known devices which measure pressure variations in fluids by electrical means. U.S. Pat. No. 3,021,711 issued on Feb. 20, 1962 to G. Arvidson for DEVICE FOR MEASURING PRESSURE OR DIFFERENCE IN PRESSURE OF FLUIDS, describes a transducer which has a pair of electromagnetic coils mounted on a nonmagnetic frame and disposed within a vibrating cylinder. The electromagnetic coils are mutually perpendicular and are transverse to the axis of the vibrating cylinder. A pair of wires lead from each coil through the outer housing of the transducer for coupling to external electrical circuitry. By supplying an electrical current to one of the coils, the cylinder may be set into vibration according to one of its natural vibration modes. As the result of this vibration, an alternating voltage of the same frequency is induced in the second coil. By using an amplifier and feedback to the first coil, a continuously oscillating system can be established about some natural frequency. In measuring the pressure of a gaseous or liquid media, the medium is introduced into the closed area on one side of the cylinder wall. The media pressure acting on the cylinder wall influences the natural frequency of the cylinder and causes it to vary in an amount nearly proportional to the change in pressure. Accordingly, by measuring this variation in frequency with a frequency meter, the pressure acting on the cylinder wall can be determined.
An improved version of this basic vibrating cylinder concept is described in U.S. Pat. No. 3,863,505 issued Feb. 4, 1975 to E. Moffatt, for VIBRATING CYLINDER PRESSURE TRANSDUCER, assigned to the same assignee as the present invention, in which the magnetic drive and pickup coil are disposed outside of the vibrating cylinder. One reason for this configuration is so that the closed integrity of the interior of the vibrating cylinder is improved. By the elimination of passageway for the wires leading to and from the coils within the vibrating cylinder, the hermetic sealing of the interior pressure chamber is enhanced. Positioning of the coils outside of the vibrating cylinder also allows a significant reduction in the overall external dimensions of the transducer since the vibrating cylinder is smaller.
One of the problems with prior art vibrating cylinder pressure transducers, particularly those in which the vibrating cylinder is fabricated from NI-SPAN-C alloy, is that they tend to be inaccurate at high temperatures, i.e., above 250.degree. F. This is because the NI-SPAN-C vibrating cylinder gradually loses its magnetic characteristics as its temperature approaches its curie temperature, the point at which a material in essence loses all of its magnetic qualities. Accordingly, when operating at these higher temperatures, the loss in the permeability and the saturation flux density of the Ni-SPAN-C vibrating cylinder is sufficiently large that the closed loop gain of a self-vibrating system incorporating the vibrating cylinder is drastically reduced. The frequency of the vibrating cylinder becomes unstable and in the extreme condition ceases to oscillate unless some form of temperature compensation has been provided.